What’s more, of women who became pregnant and didn’t miscarry in the first 22 weeks, birth rates were lower among minorities. While 85 percent of white women who carried their babies for at least 22 weeks eventually gave birth, the same was true for only about 80 percent of both Asian and Hispanic women and 76 percent of black women.

Excess weight lowered the chances of success: Fertility treatments were about 20 percent more likely to fail in obese women. Researchers have recognized this problem for a while now; not long ago, U.S. scientists reported that certain fertility drugs could help bring success rates in obese women up to equal those in normal-weight women. (See Reuters Health story of Jan 6, 2011.)

But in the current study, even among normal-weight women, pregnancy and birth rates were generally lower for minorities.

The findings, reported in the journal Fertility and Sterility, not only bolster evidence that excess weight may hinder the success of infertility treatments. They also back up previous studies showing that minority women have less success than white women.

And these latest results suggest that weight differences do not explain the racial gap, as some have speculated.

Exactly what causes the discrepancies is still unclear, according to the researchers, led by Dr. Barbara Luke of Michigan State University in East Lansing.

Studies have found that obese women, in general, have a harder time becoming pregnant than thinner women - possibly because of how excess fat affects hormone levels or low-level inflammation throughout the body.

As for racial differences in fertility treatment success, one potential explanation could be body-fat distribution, according to Luke’s team. Compared with white women, black women tend to carry more of their excess weight in the upper body, where it is believed to create more inflammation and greater health risks in general than fat around the hips and thighs.

Black women are also more likely to have uterine fibroids, which are associated with lower pregnancy and birth rates.

Luke’s team also speculates that there could be a role for vitamin D deficiency, which is more common in darker-skinned people than whites. Vitamin D deficiency has been linked to an increased risk of bacterial vaginosis, an infection that can up the chances of complications during pregnancy.

The findings are based on more than 31,000 embryo transfers performed at U.S. fertility clinics in 2007. The embryos are created by uniting eggs and sperm in a lab dish; embryo transfer refers to the process of implanting the embryos in a woman’s uterus.

White women accounted for most of the procedures - nearly 25,000.

Factors like age and the type of infertility did not explain the racial differences, but the researchers were limited in the information they had on women in the study.

That means more studies are needed to specifically address the race question in fertility treatment success, the researchers say.

“Future research,” they write, “should focus on clarifying the underlying causes of these disparities.”

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